#!/usr/bin/python
# coding:utf-8
# auther:DT
# 2017-08-13_15-03-44
sum = []
n = int(input("请输入你想查看斐波拉契的哪个数:"))
for i in range(n):
    if i < 2:
        sum.append(i)
    else:
        sum.append(sum[i-1]+sum[i-2])

print(sum[n-1])
